In this lesson you will learn how bank deposit interest gets reported in more than one way, what separates "nominal interest," "effective interest," and "real interest," and why comparing a deposit's interest rate to the inflation rate of the same period gives a truer picture of whether a saver's money actually grew or shrank in value.
Definitions
A bank deposit is an amount a person places with a bank or licensed credit institution and, under a contract, earns interest on. The principal stays with the bank, and unlike a stock or gold, its nominal amount does not fall on its own.
The nominal interest rate is the rate stated in the deposit contract or bank advertisement, usually as an "annual percentage" (for example, "24 percent per year"). On its own, this figure does not say how many times a year the interest is paid out and added to the principal.
The effective interest rate is the return once you account for how often interest is paid and added back to the principal during the year (compounding). If interest is credited monthly and itself starts earning interest the following month, the effective annual rate ends up slightly higher than the stated nominal rate.
The real interest rate is what remains after stripping out the effect of inflation: it shows how much a deposit's purchasing power, not just its nominal figure, changed over a period. The rough relationship is: real interest is approximately equal to the nominal (or effective) interest rate minus the inflation rate of the same period.
From Nominal to Effective Interest
Suppose a bank advertises a nominal annual rate of 24 percent on a deposit but actually pays that interest monthly, meaning 2 percent each month, and the depositor never withdraws it. Because each month's new interest is earned on both the original principal and the interest already added in prior months, the total interest collected by year end comes out slightly above 24 percent of the original principal; that larger figure is the "effective annual rate." The more often interest is paid during the year (daily instead of monthly, for instance), the wider, though still modest, the gap between the effective and nominal rates becomes.
From Effective to Real Interest
No matter how large it is, the effective rate only shows how much the nominal figure of the money grew. To see how much real purchasing power that growth actually delivered, the effective rate has to be compared with the inflation rate over the same stretch of time. If the effective rate is higher than inflation, real interest is positive and the deposit's purchasing power rose; if the effective rate is lower than inflation, real interest is negative, and the deposit's purchasing power fell even though interest was paid.
A Worked Example
Suppose you place 200 million tomans in a deposit with a nominal annual rate of 24 percent, paid monthly. Once monthly compounding is factored in, the effective annual rate on this deposit comes to roughly 26.8 percent, meaning you would hold about 253.6 million tomans by year end. Now consider two hypothetical cases. First, suppose inflation over the same period ran at 20 percent; real interest is then roughly 26.8 minus 20, or about positive 6.8 percent, so the deposit's purchasing power rose slightly. Second, suppose inflation over the same period instead ran at 40 percent; real interest is then roughly 26.8 minus 40, or about negative 13.2 percent, meaning that despite earning a positive nominal and effective return, the purchasing power of the whole balance fell compared with the start of the year. This example is entirely hypothetical and is meant only to illustrate the calculation.
Transmission Channel: Iran's Deposit Market
In Iran's banking system, the Money and Credit Council (شورای پول و اعتبار) periodically sets or approves ceilings on the nominal interest rate for different deposit types, from short-term daily-interest deposits to one-year, two-year, and three-year term deposits; banks are required to keep the interest they pay within those ceilings, though special plans or account combinations can offer customers a different effective return. A short-term deposit typically pays a lower, daily-accrued rate and is highly liquid; a long-term deposit carries a higher nominal rate, but withdrawing before maturity typically reduces the interest actually paid under the contract. To compare a deposit's return with inflation, the Statistical Center of Iran and the Central Bank of the Islamic Republic of Iran regularly publish monthly and annual inflation reports, introduced in this series' first lesson.
Common Mistakes
One common mistake is comparing two deposits purely on their stated nominal rates without checking how often interest is paid; a deposit with a lower nominal rate but daily compounding can end up with an effective rate close to, or even matching, one with a higher nominal rate but annual compounding. Another common mistake is assuming that earning any interest at all automatically means savings are "gaining value." As the example above showed, if inflation outpaces the effective rate, a depositor still earns a positive nominal and effective return while losing purchasing power overall.
Summary
The nominal rate is the figure stated in a deposit contract; the effective rate is that same return once compounding is factored in over the year; and the real rate is the effective return after stripping out inflation, showing the actual change in a deposit's purchasing power. This is the third lesson in the Sahmino Academy series; the previous lesson covered "What Is an Asset? A Map of an Iranian Household's Assets," which introduced bank deposits alongside a household's other assets. Future lessons will turn to the stock market and other financial instruments.
This content is for information and education only and is not investment advice. All figures carry a stated as-of date and may have changed since; verify current data from official, up-to-date sources before making any decision.
Market update
Market update
Market update
Market update